General Chemistry II Laboratory, Lab Reports of Chemistry

Download General Chemistry II Laboratory and more Lab Reports Chemistry in PDF only on Docsity! 1 General Chemistry II Laboratory Experiment #2 Inorganic Qualitative Analysis Separation and Characterization of Group A Cations Introduction: Inorganic qualitative analysis is the unambiguous identification of cations (and/or anions) which are present in a given solution. Unique tests for all ions in the possible presence of all others are virtually impossible to devise. Many tests give similar results with different cations. But if a solution is treated to separate ions into smaller groups, identification is simplified. The most common way to subdivide cations into smaller groups is by selective precipitation, in which a small group of cations is precipitated chemically. The precipitate can then be physically separated from the remaining cations in solution by centrifuging. The precipitate (insoluble solid) settles out and the solution (supernatant liquid) is decanted into another container. In this way the initial large group can be separated into smaller and smaller groups until a definitive test can be performed to verify the presence or absence of each specific cation. It is important to realize that many chemical reactions do not go to completion. The extent to which a reaction occurs depends on the magnitude of the equilibrium constant, Ksp for the reaction and the concentrations of reagents present. Equilibria can be shifted by adding or removing reagents and by altering the physical conditions in accordance with Le Châtelier's principle. In this analysis scheme, you will use various reagents to force precipitation to occur, dissolve sparingly soluble compounds or complex particular ions so they will not interfere with tests for other ions. There are several ways to classify metal ions according to the solubility of the compounds they form with various anions. We will use an abbreviated classification of ions into two groups called A and B. You will analyze an unknown solution containing from three to six of the cations in Group A (Bi+3, Fe+3, Mn+2) and Group B (Al+3, Cr+3, Sn+4) during the next two laboratory periods. The ions in an combined unknown solution will be separated into the two groups and then each portion will be analyzed for the cations present. A useful way of tabulating qualitative analysis results is with a divided page. The left column lists the test procedures, the center column the test results, and the right column the conclusions drawn from the test results. The technique is illustrated below for a solution of known ions and for a hypothetical unknown solution. Test Result Conclusions 1 Add NH3 at pH 10 to known and unknown Red precipitate formed in known and unknown Possible Group A and/or Group B cations present n both 2 Add NaOH and H2O2 to known and unknown Half of precipitate in known and unknown dissolves Possible Group A and B cations in known and unknown 3 Add hot HCl to known and unknown Precipitate in both known and unknown dissolves Mn+2, Bi+3 and Sn+2 possible in known and unknown 4 Add H2O2 and BiO3 1- to known and unknown Purple solution in known solution while unknown remains colorless Mn+4 present in known No Mn+4 present in unknown 2 Your initial unknown will contain from three to six cations from Groups A and B. You will separate the two groups and analyze Group A this week. At the next lab period, you will analyze Group B. Groups A and B cations react with NH3 (at pH 10) to precipitate as hydroxides or oxides. Group A cations react in ammonia solution according to the equations: Bi+3(aq) + 3NH3(aq) + 3H2O(l) → Bi(OH)3(s) (white) + 3 NH4 +1(aq) Fe+3(aq) + 3NH3(aq) + 3H2O(l) → Fe(OH)3(s) (red-brown) + NH4 +1(aq) Mn+2(aq) + 2NH3(aq) + 2H2O(l) → Mn(OH)2(s) (pale pink) + 2 NH4 +1(aq) Group B cations precipitate as Al(OH)3, Cr(OH)3 and SnO2. Al+3(aq) + 3NH3(aq) + 3H2O(l) → Al(OH)3(s) (white) + 3 NH4 +1(aq) Cr+3(aq) + 3NH3(aq) + 3H2O(l) → Cr(OH)3(s) (green) + 3 NH4 +1(aq) Sn+4(aq) + 2H2O(l) + 4NH3(aq) → SnO(s) (white) + 4 NH4 +1(aq) On addition of NaOH and H2O2, to the mixed group precipitate, the Group B cations will dissolve while the Group A cations remain as solids. At this point, the Group A cations can be separated physically from Group B by centrifuging and decanting. You will save the precipitate containing the Group A ions for analysis in the first week. You will save the supernatant liquid containing the Group B cations for analysis next week. Bi(OH)3 and Fe(OH)3 do not react further with either NaOH or H2O2, but Mn(OH)2 is converted to MnO2 as shown in the equation below. Mn(OH)2(s) + H2O2(aq) → MnO2(s) + 2H2O(l) The precipitate of Group A cations is dissolved in hot HCl to give a solution containing Bi+3, Fe+3, and Mn+4 ions. Some Mn+4 may be converted to Mn+2 but this will have no effect on the confirmation of manganese. The tests for each of these three cations can be carried out without any further separation. A separate aliquot of this acidic solution will be used for each of the following tests. Manganese(II) Ion: The Mn+4 ion is treated with H2O2 to convert it to Mn+2. The Mn(II) ion is treated with bismuthate ion (BiO3 -1) to form the purple permanganate ion. These reactions are shown in the following equations. The appearance of the purple permanganate color confirms Mn+2. Mn+4(aq) + H2O2(aq) → Mn+2(aq) + O2(aq) + 2H+l(aq) 14H+l(aq) + 2Mn+2(aq)+ 5BiO3 -1(aq) → 5Bi+3(aq)+ 7H2O(l) + 2MnO4 -1(aq) (purple) Bismuth Ion: In the second portion of solution, Bi+3 ion, present as Bi(OH)3, is reduced to metallic bismuth (Bio) by Sn+2. This reaction occurs in basic solution in which Sn(II) exists as the Sn(OH)3 -1 ion. The appearance of a black precipitate confirms Bi+3. The reactions are as follows: 5 Name:_________________________________ Section No:______ Qualitative Analysis Report Group A Cations Data Page for Known Solution Test Result Conclusion 6 Name:_________________________________ Section No:______ Qualitative Analysis Report Group A Cations Data Page for Unknown Solution Unknown No.______________ Test Result Conclusion 7 General Chemistry II Lab Report for Experiment #2, Inorganic Qualitative Analysis Separation and Characterization of Group A Cations Name:_________________________________ Section No:______ Unknown No.______________ 1. What Group A cation(s) are present in your unknown ? ____________ ___________ ____________ (15 points) 2. Draw a flow diagram showing the steps and products you found in the analysis of your unknown. (5 points)